Synthetic and natural speech processing by adults with or without aphasia: An investigation of attention allocation

A series of three experiments explored differences in attention allocation when adults with aphasia and adults without aphasia process natural and synthetic speech. Experiment 1 participants were 80 young adults, Experiment 2 participants were 90 adults in three age groups ranging from 20 to 79 years old, and Experiment 3 participants were 9 adults with aphasia and 9 adults without aphasia matched for age and education. In all three experiments, participants performed linguistically based single and dual tasks using natural and synthetic speech for stimulus presentation. Experiment 1 accuracy and reaction time results revealed that neurologically intact young adults have significantly greater difficulty comprehending single-syllable words presented via synthetic speech than via natural speech. Experiment 2 ANOVA computations confirmed that synthetic speech presentation resulted in significantly longer reaction times than natural speech presentation and that younger adults performed the experimental tasks significantly more rapidly than older adults. Experiment 3 results revealed significant differences in the processing speed of adults with aphasia and adults without aphasia after adjusting for motor speed differences through use of a covariate. Response speed discrepancies between participant groups increased with task complexity. Furthermore, adults with aphasia performed with substantially greater response speed variability than adults without aphasia across all tasks, but especially when listening to synthetic speech. These results support the notion that persons with aphasia are inefficient in their allocation of attention resources, and the findings have implications for using synthetic speech devices when comprehension is paramount.